|No, not you.|
First, why did I pick Weasel of Doom as an example? Simple. I love the blog's name (see picture above), whoever runs it reads an obscene number of books, one of those books is a Max Barry book, and the blog gives an A to the works of Guy Gavriel Kay.
An abridged version of the rating system is as follows:
Five stars – Excellent (A). If I were told to pick books to take with me to a desert island, these are the ones I would take.
Four stars – Good (B). Would recommend to others.
Three stars – Average (C). Writing was enjoyable, time went by quickly, but I am not seized with desire to run out and buy the author’s back list.
Two stars – Bad (D). Either the writing was clunky, or plot holes abounded, or protagonists were not likable (or all of the above).
One star – Aweful (F). Writing is horrible, plot non-existent, protagonists inspire hatred.
The presence of tens of thousands of great books in the world leads me to (hopefully) avoid the D and F categories. I freely admit that any attempt to rate all the newest books, rather than established classics, inevitably results in a lower overall average. This is where the key difference makes itself seen.
When rating new media, unless you're evaluating an established artist at his/her creative peak, there are bound to be ups and downs. This is where a traditional star rating system is best. It started with short story reviews but has since been overshadowed by movie reviews. Determining which book to buy out of the four new releases that haven't gone on sale yet and thus are excruciatingly expensive is when quality ratings help someone. Weasel of Doom's looks good enough, basically a retread of the classic 1-5 star system, although I'd like to see more pluses and minuses. At the top end, there's a large gap between an instant legend and a merely "good" book. I could see a B+ or A- handed out a fair bit, so it'd be interesting to see Weasel of Doom tap that space a little more.
When rating classic media,** presumably someone liked it. Otherwise, it would fall into obscurity with barely a trace. This is the reason oldies stations play a combination of their eras' greatest hits and most critically acclaimed songs. When people go back even farther, to times so old the art is more of an archaeological artifact than anything that could reasonably be called pop culture, it's less about whether the work is good. Evaluation then becomes about noting the influence it's had on later works, what sorts of tastes go well with it, and how a modern reader can relate to it. Sure, I think Julius Caesar is better than Othello, but how is that any use to the tenth-grader who's been assigned one or the other? I haven't read a Shakespeare play since 2010, and I have an odd tendency to read Elizabethan literature.
NOTE: "Protagonists inspire hatred", from Weasel of Doom's rating system, is a more ambiguous phrase than it seems. Do they inspire hatred because that was the author's goal, or are they merely unlikable? A character like Jason Compson inspires hatred quite effectively, regardless of who the actual protagonist of The Sound and the Fury is. A character like Bella Swan, according to these reviewers, inspires the type of hatred that makes me glad I haven't read Twilight.
As with many breakable rules of writing, the answer to the question "Is the author achieving what s/he set out to achieve?" is a good standard here. It's also the standard I try to use when rating anything, including when approaching works for which I have no compass due to moral, cultural, political or other reasons. Do I, a decided non-linguist, give a linguistics book four stars or four and a half? I don't know. All I can do with a book so far outside my area of knowledge is say how easy a read I think it is and how much I think someone can learn from it. Books are communication - I'm here to evaluate the relaying of the message.
*I try to avoid rating books on their merits. Instead, as you can see from my Educational Content and Ease of Reading ratings, I tailor the books I've read to the audience that would enjoy them the most.